Two common methods of auto-focus detection and adjustment for an image sensing apparatus using a light flux that passes through the photographing lens are the contrast detection method (also called the blur method) and the phase difference focus detection method (also called the shift method).
In the contrast detection method, which is widely used in video cameras for moving image photography and in electronic still cameras, the image sensor is used as a focus detection sensor. In this method, of the output signals of the image sensor, information of the high frequency component (contrast information) in particular is concentrated on, and the position of the focus lens at which an evaluation value for that contrast information is at its maximum is taken as the focus position. However, as it is also called the mountain-climbing method, this method involves getting evaluations of the contrast while moving the focus lens in minute amounts and thus requires moving the lens until it is determined which evaluation was in the end the maximum, and therefore is unsuitable for high-speed focus control.
The phase difference focus detection method is widely used in single-lens reflex film-type cameras, and is the technology that has contributed the most to the commercialization of the AutoFocus (AF) single-lens reflex camera. In the phase difference focus detection method, a light flux passing through the exit pupil of the photographing lens is split in two, with the split light fluxes respectively received by a set of focus detection sensors. By detecting an amount of shift in signals that are output according to the amount of light received, that is, by detecting the amount of shift in relative positions in the direction of split of the light flux, the amount by which the photographing lens is out of focus can be obtained directly. Therefore, once charged by the focus detection sensors, the amount by and direction in which the lens is out of focus is obtained, making high-speed focus control possible. However, in order to split the light flux passing through the exit pupil of the photographing lens in two and obtain signals for each of the split light fluxes, typically a light path split mechanism, such as a quick-return mirror or a half-mirror, is provided in the image sensing light path, with a focus detection optical system and an AF sensor at the end. As a result, the apparatus increases in size as well as cost.
At the same time, however, CCD and CMOS image sensors are now widely used in place of film as the image sensing medium even in single-lens reflex cameras as well. As a result, models have now appeared that are equipped with an electronic viewfinder mode realized by withdrawing the quick-return mirror from the image sensing path so as to display an object on a display device provided in the camera body and enable the user to view the object and a moving image recording mode that simultaneously records moving images on a recording medium.
There is a drawback in this case, in that a focus detection apparatus of the phase difference focus detection method described above, because the quick-return mirror is withdrawn, cannot be operated.
To solve the above-described drawback, a technology that provides the image sensor with a phase difference detection capability, thus eliminating the need for a dedicated AF sensor and achieving high-speed phase difference AF, has also been disclosed.
In Japanese Patent Application Laid-Open (JPA) No. 2000-156823, a pupil dividing function is provided at a portion of the light-receiving elements (pixels) of the image sensor by shifting a sensitive area of the light-receiving part with respect to an optical axis of an on-chip microlens. Then, these pixels are used as focus detection pixels, which are disposed at predetermined intervals between image sensing pixel groups to carry out phase difference focus detection. In addition, since the locations at which the focus detection pixels are disposed correspond to image sensing pixel loss parts, they are interpolated using adjacent image sensing information to generate image information.
Japanese Patent Application Laid-Open (JPA) No. 2000-292686 provides a pupil dividing capability by dividing the light-receiving part of a portion of the pixels of the image sensor in two. These pixels are then used as focus detection pixels, which are disposed at predetermined intervals between image sensing pixel groups to carry out phase difference focus detection. In addition, in this approach as well, since the locations at which the focus detection pixels are provided are locations where image sensing pixels are lost, they are interpolated using adjacent image sensing information to generate image information of the lost pixels.
Japanese Patent Application Laid-Open (JPA) No. 2001-305415 provides a pupil dividing capability by dividing the light-receiving part of a portion of the pixels of the image sensor in two. Then, the outputs from the light-receiving part divided in two are processed individually to carry out phase difference focus detection, and by adding together the outputs from the light-receiving part divided in two are also used as the image sensing signal.
However, the above-described known art has the following drawbacks.
In the art disclosed in Japanese Patent Application Laid-Open (JPA) No. 2000-156823, Japanese Patent Application Laid-Open (JPA) No. 2000-292686, and Japanese Patent Application Laid-Open (JPA) No. 2001-305415, there are differences in the light-receiving characteristics, such as the light-receiving part surface area of the focus detection pixels is small compared to that of the image sensing pixels, or the center of gravity of the light-receiving area is shifted with respect to the optical axis of the on-chip microlens. As a result, at the location where the focus detection pixels are disposed the pixel information is lost, or, because it has a large error if used as image information, it is necessary to interpolate from the adjacent image sensing pixels to generate the image signal.
Accordingly, if the disposition density of the focus detection pixels can be reduced, the deterioration in picture quality due to pixel loss as described above can be reduced, but at the cost of degrading focus detection image sampling characteristics and reducing focus detection performance. In other words, in this technical field, in order to both maintain focus detection performance and prevent picture quality deterioration, the method of disposition of the focus detection pixels is very important. However, in Japanese Patent Application Laid-Open (JPA) No. 2000-156823, Japanese Patent Application Laid-Open (JPA) No. 2000-292686, and Japanese Patent Application Laid-Open (JPA) No. 2001-305415, the focus detection pixels are concentrated in one area. As a result, the focus detection area is restricted to a specific location in the image sensing area, and in that area picture quality degradation tends to be most easily noticeable because of the low distribution density of image sensing pixels thereat.
In addition, in the electronic viewfinder mode and the moving image mode, smooth screen display is very important. Moreover, and also because resolutions higher than those of still images are not required, typically, the output of the image sensor is often subjected to thinning readout to produce the image in order to increase the frame rate.
With the disposition of the focus detection pixels described in Japanese Patent Application Laid-Open (JPA) No. 2000-156823, in a case in which the image sensor is subjected to thinning readout, the focus detection pixels are not read out, and the phase difference focus detection method cannot be carried out.
In the inventions described in Japanese Patent Application Laid-Open (JPA) No. 2000-292686 and Japanese Patent Application Laid-Open (JPA) No. 2001-305415 as well, because the focus detection pixels are concentrated in one area, in a case in which thinning readout is carried out, the focus detection pixels are thinned. As a result, sampling characteristics deteriorate and focus detection performance deteriorates dramatically.